Wafer fabrication operations for producing silicon wafers suitable for use with microprocessors, computer memory, and other microcircuits must be carried out in a substantially contaminant-free environment. Airborne particles such as dust and dirt can compromise the quality of the resulting wafer. Excessive foreign matter results in a lower yield of usable wafers from a particular batch. Increasingly dense arrangement of circuit elements on a wafer, or chip, further heightens the quality required to produce a usable wafer. Accordingly, such fabrication operations are often performed in a sealed, substantially contaminant-free processing environment.
Transport of these wafers to and from such a processing environment requires access through an opening in a wall to a sealed chamber in which wafer processing occurs. This processing Express Mail Number environment, referred to as the tool side of the wall, contains tools and apparatus for effecting such processing. On the side opposite the processing environment, referred to as the operator side of the wall, transport of the wafers between processing stations occurs by human or robotic operators.
The opening in the wall must be capable of being sealed sufficiently by a door assembly to maintain the contaminant-free state on the tool side, but also be readily unsealed to provide expedient and unobstructed access between the operator and tool sides when needed. Such a seal between the door assembly and the opening often involves close tolerances between moving parts, which raises issues of friction and inaccurate sealing, as wear-and-tear affects the seal mechanism.
The wafers are often stored in a sealed cassette called a pod when they are not in the processing environment on the tool side. Such pods are designed to engage and seal against the opening in the wall from the operator side to allow the wafers to be accessed by a mechanism from the tool side while maintaining the separation between the processing environment and the operator side environment. This separation is achieved by abutting the pod in sealing engagement with the operator side of the wall adjacent the opening therein. Then, the pod door is engaged by the door assembly, unlocked from the pod, and drawn by the door assembly into the tool side. In this manner, the outer perimeter of the pod opening remains sealably engaged with the outer perimeter of the opening in the wall. With the pod door engaged by the door assembly and retracted from the pod, the wafers inside the pod are accessible for processing by apparatus on the tool side. After processing, the wafers are returned to the pod and the door assembly is moved to reengage the pod door with the pod, thereby also closing the opening in the wall with the door assembly. Further, the pod must remain undisturbed when the pod door is open to the processing environment, as manipulation of the pod, such as accidental removal, could disturb the seal and cause physical damage to the wafers.
Automation of such a wafer handling and treatment operation is therefore burdened by the need to manipulate the door assembly and pod door within these close tolerances, the need to accurately reengage with and reseal the pod door and wall opening following wafer treatment, and the need to secure the pod during treatment to avoid accidental removal. Movement of a door assembly concealing such an opening, however, is typically along two orthogonal axes. Alternatively, a pivoting mechanism is used to effect movement along a single arcuate path.